Purification of acetaldol by distillation



Feb. V6, 1945. K. H. w. TUERCK ETAL' ,3

PURIFICATION OF ACETALD OL BY DISTILLATION Filed March 6; 1944 QETURN I 4' Inventor Karl H-NT-x a; S. Lam-a3.

Attorney .kept cool.

Patented Feb. 6, 1945 PURIFICATION OF ACETALDOL BY DISTILLATION Karl lieinrich Walter Tuerck, Banstead, and Hans Joachim Lichtenstein, London, England, 'assignors to The Distillers Company Limited,

Edinburgh, Scotland, :1. British company Application March 5,1944, Serial No. 525,232

- In Great Britain February 24, 1943 10 Claims. ('01. 202-48) In order to obtain pure acetaldol, crude acetaldol, as obtained by the condensation of acetaldehyde with alkali and which contains free acetaldehyde and higher polymerised products, is generally distilled. During this distillation, even if it is carried out under high vacuum, a considerable part of the acetaldol resinifies, and high molecular weight unsaturated aldehydes, as wel1 as crotonaldehyde, are formed and this is particularly noticeable when distilling technical acetaldol which contains a considerable proportion of acetaldehyde. These substances exercise an adverse effect on the reactions for which acetaldol is used as starting material. Acetaldol of high purity can be obtained, however, in accordance with the process of the present invention which comprises distilling crude acetaldol in vacuo by leadingit into a distillation column in countercurrent to the vapours of a normally liquid hydrocarbon, which is immiscible with acetaldol and has a boiling point of from 1'70-300'C. at normal pressure, condensing the distillate and separating the purified acetaldol layer from the hydrocarbon layer. The distillate separates into two layers, the lower one being water-clear monomeric acetaldol which will polymerise to paraldol unless it is The hydrocarbon, which forms the upper layer, may be returned to the still from which the vapours rise into the column. The vacuum in the column during the distillation should be maintained at a level which enables a temperature to be maintained at the top of the column not above 90 C. preferably between Ill-80" C.

In carrying out the invention, we prefer to introduce the crude acetaldol gradually at a point near the top of the column, whereas the hydrocarbon is evaporated in a separate still and its vapours introduced near the bottom of the column; the residues from the distillation are run oil, at the bottom of the column. If the residues volatile aldehydes taking place and the distillate appears more or less coloured.

Crude acetaldol which normally contains about 50% by weight of acetaldehyde may be'subjected as such, to the distillation process of the present invention, or-it may first be freed from the major part of the acetaldehyde by lmown methods. Free acetaldehyde passing from the distillation column may be condensed behind the vacuum pump at normal or increased pressure.

The following example illustrates the manner in which the invention maybe carried into effect.

Through a packed column, dodecane was distilled at a. pressure of 25 mm. of mercury, while at the top of the column acetaldol, containing about 20% by weight of acetaldehyde, was introduced. The temperature at the top of the still was kept at 75 C. In the condenser a mixture of 3 parts by weight of dbdecane and 1 part by weight of acetaldol condensed, whereas in the bottom of the column a small amount of a viscous residue remained. From 450 grams of crude acetaldol, 350 name of pure, water-clear acetaldol were collected as distillate. The two layers in'the dis tillate were separated and the hydrocarbon layer was returned to the still at the bottom of the col- When acetaldol from the same sample was distilled at the same pressur directly from an extion can be carried out.

As will be seen this apparatus comprises a I packed column, to which reference is made in the do not run off separately, but back into the hydrocarbon still, care should be taken that the high-boiling materials form only a small part, preferably not more than 1 6, of the volume or the hydrocarbon in the still. The present process may be applied with great advantage to acetaldol,- containing acetaldehyde, which has been stored for some time and in which a high proportion of higher polymers are present. These higher polymers are depolymerised during the distillation in the column without the formation or resinous foregoing example, and this column is provided with an input near to the base of the column for the vapours generated in the still by the action of the heater.

The hydrocarbon vapours of course flow up the L column in countercurrent flow to the crude acetalwhich connects with the condenser and in this matter, whereas, if distilled from a kettle, the major part cannot be evaporated at all without resinmcation or decomposition to unsaturated condenser the mixture of the hydrocarbon and acetaldol are condensed. The purified acetaldol is withdrawn from the condenser through the draw-oi! cock shown whilst the upper layer 01 hydrocarbon is withdrawn through the pipe indicated and conveyed to the still.

Thewholesystemis-maintainedunderreduced pressure by means of the vacuum pump which is shownconnected to the condenser.

The column is shown effected with a draw-off cock at the base and through this the high boiling residues passing downwardly through the column can be withdrawn and if desired these can be returned to the still provided that, the amount of high boiling materials being fed into the still does not exceed 10% by weight of the hydrocarbon in the still; alternatively, the high boiling residues can be withdrawn from the base of the column and separately dealt with. The apparatus is of course adapted for continuous operation.

A special advantage of our process is that it allows of the use of a vacuum of 10-30 mm. (which is conveniently attainable commercially) and eliminates the detrimental eflect which fluc tuations in pressure have in the usual straight vacuum distillations of acetaldol.

What we claim is: V

1. A process for the purification of crude acetaldol which comprises distilling crude acetaldol in vacuo by leading it into a distillation or fractionating column in countercurrent to the vapours of a normally liquid hydrocarbon, which is immiscible with acetaldol and has a boiling point of from 170-3 00 C. at normal pressure, condensing the distillate and separating the purifled acetaldol layer from the hydrocarbon layer.

2. A process according to claim 1 wherein the pressure in the said column is so selected that the temperature at the top of the column is'not.

above 90 C.

- 3. A process according to claim lwherein the pressure in the said column isso selected that the-temperature at the top of the column is 70". to 80C. I

4. A process according to claim 1 wherein the hydrocarbon layer. which is separated off is returned to a still in which the said vapours are generated.

5. A process according to claim 1 wherein the residues from the distillation column are run oif separately.-

6. A process according to claim .1 wherein the residues from the distillation column are returned to a still in which the hydrocarbon vapours are generated, the amount of high-boiling materials being controlled to a value not in excess of 10% of the volume of the hydrocarbon therein.

'7. A process according to claim 1 wherein the said hydrocarbon vapours are introduced into said column near to the base thereof and the crude acetaldol introduced into said column near to the top thereof.

8. A process according to claim 1 wherein the operating pressure of said column is maintained between 10 and'30 mm. of mercury.

9. A process according to claim 1 wherein the crude acetaldol is freed from the major part of the co-present acetaldehyde prior to being introduced into said column.

10. A continuous process for the purification of crude acetaldol which comprises generating the vapours of a normally liquid hydrocarbon, which is immiscible with acetaldol and has a boiling point between 170 and 300C. under normal pressure, in a still, passing said vapours into a packed column maintained at reduced pressure, introducing crude acetaldol into said column, transferring a mixture of the vapours of said hydrocarbon and acetaldol from the top of the column to a condenser, separatingthe condensate into two fractions consisting respectively of purified acetaldol and said hydrocarbon, recover ing the purified acetaldol and recycling said hydrocarbon fraction to the still.

KARL. HEINRICH WALTER. TUERCK.

HANS JOACHIM LICHTENSTEIN. 

